The biliary tree descends from the canaliculi at the hepatocytes, gradually enlarging and merging to the right and left hepatic ducts.
These 2 merge at porta hepatis to form the Common Hepatic Duct(CHD).
The Common Bile Duct (CBD) is formed by the junction of the cystic duct with the CHD.
The CBD traverses through the head of the pancreas entering the duodenum at the Ampulla of Vater through the Sphincter of Oddi.
Prior to draining into the duodenum the CBD is joined by the pancreatic duct.
A smaller accessory pancreatic duct and sphincter is usually present (but rarely visible) from the dorsal pancreatic embryologic rotation.
Anatomy of the Biliary System
A common anatomic variation is to have an extrahepatic junction of the right/left hepatic ducts.(20% of people) It is useful to make note of this in the report, particularly if it is a very distal junction. During cholecystectomy surgery, the common duct is cannulated to image for duct calculi which may be missed. Also to avoid mistaking the RHD for the cystic duct.
A less common, but important variation, is for the right hepatic duct to join the cystic duct rather than merge with the LHD.
Extrahepatic Junction ultrasound image.
Ultrasound image- A common variant is the common bile duct coursing below the hepatic artery. In colour (mouse over) it proves the hepatic artery (in blue) is anterior.
Role of Ultrasound
Ultrasound is the primary tool for assessment of the structure of the biliary tree. Whilst studies on have been performed on functional assessment of CBD diameter (pre/post fatty meal), they are not easily or objectively reproducable. Physiological assessment of gallbladder function is best performed with Nuclear medicine or CT cholangiography.
Follow-up of a cbd stent and other surgical interventions.
Post surgical complications eg abscess, biloma
Guidance of injection, aspiration or biopsy
Non fasted patients
Large habitus patients
The patient should be fasted.
Use of a curvi-linear probe 3-Mhz depending on patient habitus.
If the gallbladder is superficial in a thin patient, a linear array may be utilized however reverberation from the anterior abdominal wall becomes an issue. To minimze this artefact, try reducing transducer pressure
Good colour / power / Doppler capabilities when assessing vessels Versus ducts or vascularity of the gallbladder wall.
Be prepared to change frequency output of probe (or probes) to adequately assess both superficial and deeper structures.
Throughout a comprehensive investigation the patient may need to be supine, erect or left decubitus.
Utilise inspiration/expiration and asking tthe patient to ‘puff their stomach out’ like a pregnant belly. This can move anatomy out from beeath the ribs and help mobilize bowel gas obscuring your view.
The source of obstruction may be:
Tumour extrisic compression
Collection (post surgical/infection)
The patient should be fasted.
Begin with the patient supine.
Assess the pancreas and visualise the Common Bile Duct (CBD) in the head of pancreas.
Follow the CBD back into the liver at porta hepatis.
Measure the diameter of the extrahepatic bile duct.
Basic Hardcopy Imaging
Image the CBD from the liver to the ampulla.
Measure it in the widest and follow it to the thinnest portion extra-hepatically.